1. Field of the Invention
This invention relates to communication systems and particularly (though not exclusively) to Time Division Duplex (TDD) operation in radio communication systems.
2. Description of the Related Art
In the field of this invention it is known that power management of Forward Access Channel (FACH) transport channels may be employed in a Universal Mobile Telecommunication System (UMTS) radio communication system.
In a 3rd Generation Partnership Project (3GPP) radio communication system, the FACH is a common transport channel used for downlink transmissions. Data destined for a number of User Equipment units (UEs) may be mapped onto FACH(s). The FACHs are then multiplexed by the physical layer, mapped onto Coded Composite Transport Channels (CCTrCHs), and transmitted on Secondary Common Control Physical Channel (S-CCPCH) physical channels. UEs receive the S-CCPCH physical channels, extract the FACH(s) and identify data that is intended for them. Typically the data to be sent on the FACH(s) is rescheduled every Transmission Time Interval (TTI) (where TTI=10, 20, 40 or 80 ms).
The concept of applying power management on the FACH is known in the prior art. For example, in published US patent application no. US 2002/0094833 A1 it is known that the transmit power is regulated according to:
UE Measurements                received signal strength        SIR (Signal to Interference Ratio)        Error rates (Block Error Rate—BLER, Bit Error Rate—BER)        
Current Conditions in the Cell                traffic volume        % max BTS (percentage of maximum Base Transmitter Station) power in use.        
Service Requirements of the UE.
In document “MBMS Power Usage” (available from the 3GPP website at www.3gpp.org/ftp/tsg_ran/WG2_RL2/TSGR2_31/Docs/Zips/R2-022110.zip) and document “Comparison of DSCH and FACH for MBMS use” (available from the 3GPP website at www.3gpp.org/ftp/tsg_ran/WG2_RL2/TSGR2_31/Docs/Zips/R2-022110.zip) power control of the FACH has also been suggested in 3GPP for broadcast usage (MBMS, Multimedia Broadcast Multicast Service). Here the suggestion is made that if the network knows the geometry of the MBMS users in the cell, the transmission power can be set just enough to serve all users subscribing to the MBMS service instead of blindly using the power to cover the whole cell.
However, a disadvantage of these and similar prior art schemes is the difficulty of estimation of the power needs of individual UEs.
Furthermore, the required measurement and processing operations of the UEs result in increased complexity and increased communication of data over the air interface.
Also, a simple power control operation may not result in optimal resource usage in a communication system. Thus, conventional systems may have reduced capacity.
Hence, an improved method and apparatus for power control in a radio communication system would be advantageous and in particular a system wherein the abovementioned disadvantage(s) may be alleviated would be advantageous.